The present invention relates to a magneto-optical recording medium for use as a magneto-optical memory, a magnetic recording and display cell and so forth and, more particularly, to a magnetic thin film recording medium which has an easy axis of magnetization in a direction perpendicular to the film surface and permits recording of information by forming a reversed magnetic domain of a circular or any other arbitrary configuration and readout of the information through utilization of a magnetic-optical effect such as the magnetic Kerr effect.
With ferromagnetic thin films which have an easy axis of magnetization in a direction perpendicular to their film surface, it is possible to form a small reversed magnetic domain of a magnetic polarity reverse to the uniform magnetization polarity in the film surface uniformly magnetized to the south or north magnetic pole. By making the presence and absence of such a reversed magnetic domain correspond to states "1" and "0", respectively, such ferromagnetic thin film can be employed as high density magnetic recording media. Of such ferromagnetic thin films, those which have a large coercive force at room temperature and a Curie temperature or magnetic compensation temperature relatively close to room temperature permit recording of information by forming reversed magnetic domains at arbitrary positions with a light beam through utilization of the Curie temperature or magnetic compensation temperature, and they are generally used as beam-addressable files.
Conventionally known ferromagnetic thin films which have an easy axis of magnetization in the direction perpendicular to the film surface and can be used as beam-addressable files are polycrystalline metallic thin films represented by MnBi, amorphous metallic thin films such as Gd-Co, Gd-Fe, Td-Fe, Dy-Fe, etc., and compound single crystal thin films represented by GIG; however, they have such advantage and disadvantage as described below. The polycrystalline metallic thin films which utilize the Curie temperature for a writing operation, represented by MnBi, are excellent as magnetic recording media, in that they have a large coercive force of several kilooersteds at the room temperature, but are defective in that they call for a great amount of energy for a writing operation because of their high Curie temperature (T.sub.c =360.degree. C. in MnBi). Moreover, since polycrystalline metals are used, these thin films must be formed to have a stoichiometric composition, which introduces technical difficulties in their fabrication. The amorphous metallic thin films which effect a writing operation through utilization of the magnetic compensation point, such as Gd-Co and Gd-Fe, possess advantages; that they can be formed on an arbitrary substrate since amorphous materials are used, and their magnetic compensation temperatures can be freely controlled to some extent by the addition of a small amount of impurity, but these thin films have a shortcoming in that their coercive forces at the room temperature are small (300 to 500 Oe), resulting in recorded information being unstable. In addition, it is necessary, for the fabrication of thin films of such a small coercive force, to control their composition within about 1 atom%, and hence these thin films are not easy to fabricate.
Further, the compound single crystal thin films, represented by GIG, have the serious disadvantage of very high manufacturing costs as compared with the other thin films.
On the other hand, amorphous alloy thin films containing 15 to 35 atom% of Tb or Dy, such as TbFe and DyFe, which have been proposed as new magnetic thin film recording media free from such defects as described above, have the following merits:
(1) Since each one of them has an easy axis of magnetization in the direction perpendicular to the film surface and has a large coercive force of several kilooersteds at room temperature, information can be recorded with high density and the recorded information is very stable.
(2) The coercive force is large and magnetic domains of desired configuration can be established.
(3) Since each one of them has a large coercive force over a wide range of composition and has excellent characteristics as recording media over a wide range of composition, they need not be severely restricted in composition and can be fabricated with ease and with good yield.
(4) Since the Curie temperature TbFe is as low as 120.degree. C. and of DyFe is as low as 60.degree. C., a thermal writing operation utilizing the Curie temperature can be effected with a very small amount of energy.
However, these amorphous alloy thin films present a problem of thermal stability as they are amorphous. Amorphous, material has a metastable phase in terms of energy, but when the temperature rises, it undergoes an irreversible transformation into a crystalline state. The temperature at which it becomes crystalline is referred to as the crystallization temperature T.sub.cry. The thin film, once crystallized, comes to have an easy axis of magnetization in the direction along its surface and loses the function of an optical magnetic recording medium. Accordingly, in case of writing information in the magneto-optical memory, a temperature above the Curie temperature T.sub.c is needed, but the temperature should not be raised above the crystallization temperature T.sub.cry. This imposes severe limitations on the power of light for a writing operation. For example, TbFe has a Curie temperature nearly equal to 130.degree. C. and a crystallization temperature of 220.degree. C., and hence there is a relatively large temperature difference (a margin) therebetween. In recording media of GdTbFe systems or TbFeCo systems Gd or Co may be included for the purpose of improving their reproducing characteristics; however, the Curie temperature rises to the vicinity of 200.degree. C. and the crystallization temperature undergoes no appreciable change, leaving a very narrow margin of recording light power.
Furthermore, crystallization of the recording medium is not always caused by single light irradiation for a writing operation; in a case where the recording medium is repeatedly used, heat from the light irradiation for each writing operation is accumulated, so that even if the recording medium has a low Curie temperature T.sub.c, its temperature would exceed its crystallization temperature T.sub.cry, resulting in the medium being crystallized.
These properties are serious defects when the abovesaid thin films are employed as media of the magneto-optical memory.